Out of Plane Flexural Behavior of Brick Wall_2006

Composites: Part B 38 (2007) 559–574 www.elsevier.com/locate/compositesb

Out-of-plane flexural behavior of unreinforced red brick walls strengthened with FRP composites Ayman S. Mosallam Civil and Environmental Engineering Department, University of California, Irvine, CA 92604, USA

Received 20 March 2006; accepted 27 July 2006 Available Available online 27 December December 2006

Abstract

This paper presents the results results of a study focused focused on evaluating evaluating the out-of-plan out-of-planee flexural flexural behavior behavior of two fiber reinforced reinforced polymer polymer (FRP) composite systems for strengthening unreinforced red brick masonry walls. The full-scale tests followed the International Code Council Evaluation Service (ICC-ES) AC 125 procedure. In the experimental program, a total of four full-scale destructive tests were conducted on UMR red brick walls. One wall specimen was used as control (as-built) specimen without composites, and the remaining three wall specimens were strengthened with either E-glass/epoxy or carbon/epoxy composite systems with different fiber architecture. The effect of applying a cross-ply laminate on the ultimate failure mode has been investigated. Full-scale experimental results confirmed the effectiveness of the FRP composite strengthening systems in upgrading the out-of-plane flexural structural performance of URM walls. In addition, an analytical model was developed to predict the ultimate load capacity of the retrofitted walls. The analytical modeling is based on deformation compatibility and force equilibrium using simple section analysis procedure. A good agreement between the experimental and theoretical results was obtained.  2007

Elsevier Ltd. All rights reserved.

Delamination; D. Mechanical Mechanical testing; E. Lay-up (manual); Infrastructure Infrastructure Keywords: A. Laminates; B. Delamination;

1. Introduction

In general, unreinforced masonry (URM) buildings perform poorly in earthquakes. Two types of failure are commonly encountered encountered in URM buildings subjected to seismic forces. The first failure mode occurred in in-plane shear that are designed to form the lateral load resisting system of the building. The other type of failure is due to out-ofplane plane bendin bendingg stresse stressess caused caused by seismic seismic inertia inertiall forces forces.. The excessive out-of-plane bending is also a major reason for for the the loss loss of load load carry carrying ing capa capacit cityy of URM URM walls walls.. Fig. 1 shows 1 shows a typical failure of unreinforced red brick wall due to excessive excessive out-of-plane out-of-plane seismic forces. Composites Composites offer an attractive strengthening protocol for existing and historical unreinforced masonry structures.

E-mail address: [email protected] 1359-8368/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesb.2006.07.019

In the past few decades, composites have successfully been used in different construction applications including streng strengthe thening ning of reinfo reinforce rced d concre concrete, te, steel steel and timber timber structures. An in-depth review of different applications of compos composites ites in repair repair and rehabil rehabilita itation tion is discusse discussed d by Mosallam [1] [1].. Lately, several studies have been conducted on evaluating the use of polymeric composites for repair and and stre streng ngth then enin ingg both both unre unrein infor force ced d and and rein reinfor force ced d masonry walls subjected to seismic, wind and lateral earth pressure. The advantages of using composite materials in this application application are (i) ease of application, application, (ii) preservation preservation of the geometrical and architectural details of the walls, (iii) their high strength-to-weight ratio, and (iv) their high resistance to corrosion as compared to metallic strengthening systems. This paper presents a summary of experimental and theoretical results of a study that was conducted to characterize the out-of-plane flexural behavior of unreinforced forced masonr masonryy walls walls extern externally ally streng strengthe thened ned with fiber fiber reinforced reinforced polymeric polymeric (FRP) composite composite laminates. laminates.

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Fig. 1. Failure of unreinforced red brick wall due to out-of-plane seismic forces [Nisqually Earthquake, 28 February 2001].

Almusallam [11] conducted a study on the behavior of unreinforced masonry strengthened with composites. Several wall specimens were subjected to out-of-plane and in-plane loads. As reported, a significant strength increase was observed for all strengthened wall specimens. Turco et al. [12] evaluated both the flexural and shear strengthening of un-reinforced masonry using FRP bars. The results of the study indicated the potential of this technique for masonry strengthening applications. Similar studies were conducted by other researchers (e.g. El-Dakhakhni et al. [13], Hamoush et al. [14], Hamilton and Dolan [15], Laursen et al. [16]). Ghobarah and Galal [17] studied the out-ofplane behavior of FRP strengthened masonry walls with openings. Recently, Korany and Drysdale [18] developed an unobtrusive composite rehabilitation technique using flexible carbon/epoxy cables, mounted near the surface of the façade walls in epoxy-filled grooves in the bed and head joints.

2. Related work 3. Objective and motivations

The use of composites in strengthening masonry started initially at the Swiss Federal Laboratories for Materials Testing and Research (EMPA) in Du¨bendorf, Switzerland. One of the pilot studies in the area was reported by Schwegler [2]. Based on the results of this pilot study, a testing program on load-bearing masonry walls of a six-story building strengthened with carbon/epoxy laminates was performed [3]. Gilstrap and Dolan [4] reported the results of an experimental study focused on evaluating the structural behavior of unreinforced masonry walls strengthened externally with different types of composites. Both smalland large-scale tests were conducted with varying boundary conditions. The walls were tested under both line and concentrated uniform loading conditions. Albert et al. [5] conducted a similar experimental investigation on the feasibility of using polymeric composites as an external strengthening system for masonry walls. In their study, the performance of both undamaged and slightly damaged wall specimens was evaluated. Several parameters were used in their study including composite type, amount of applied composite, fiber architecture of the overlay and the loading regime. Ganz et al. [6] studied four types of composites systems for wall strengthening, namely: chopped E-glass/epoxy, chopped E-glass/polyester, E-glass composite fabric cloth with epoxy resin, and E-glass fabric cloth with polyester resin. The results of the study showed significant non-linearity in load–deflection relationships due to the effect of delamination between the composites and the masonry. The behavior of tested strengthened masonry beams with was studied analytically [7]. The cyclic flexural behavior of masonry walls reinforced with glass/ epoxy composites was investigated by Velazquez-Dimas et al. [8] and Kuzik et al. [9]. Tan and Patoary [10] conducted a large experimental program on 30 masonry walls strengthened using three different fiber-reinforced polymer (FRP) systems. However, the loading regime that was used was concentrated on a portion of the wall. Al-salloum and

The majority of historical structures including buildings, arches, bridges and chimneys that requires immediate repair and/or strengthening are made of red clay bricks with low-strength mortar. To date, limited work has been published on the behavior of red brick walls retrofitted with FRP composites. In addition, several studies highlighted the major influence of the loading pattern on the ultimate performance of laboratory-tested wall panels. Hence, it is critical to accurately simulate the inertial forces generated by seismic activities that are responsible for the excessive out-of-plane forces applied to the masonry walls. Line- or concentrated loading of a wall specimen produces stress concentration fields that accelerate the strength degradation of the weak aging mortar lines resulting in inaccurate laboratory-simulated performance and premature failure as compared to the actual field performance. The premature failure of the mortar lines due to the application of line and/or concentrated loads generates high shear stresses on the laminate. This shear stress concentration adversely affects the strength of the composite laminate and increases the possibility of initiating premature local laminate fracture and/or debonding at these locations. In order to accurately simulating the applied loads generated from seismic action, and avoid these potential premature failure modes, a uniform hydrostatic pressure was employed in this study. In most cases, both in-plane shear and out-of-plane flexural upgrades are required to upgrade the seismic performance of old and historical unreinforced masonry structures. In order to fulfill these demands, multidirectional composite systems are required (e.g. cross-ply, angle-ply or quasi-isotropic lamination) to achieve optimum retrofit design. To date, no work has been published to evaluate the coupling effect of composite reinforcements applied in the different directions for each demand. This effect issue is investigated and discussed in this paper. In


addition, this is one of the first studies where all test procedures are confirmed with the requirements of the International Code Council-Evaluation Service (ICC-ES) Acceptance Criteria AC 125 [19]. 4. Experimental program

4.1. Wall specimens

A total of four unreinforced red bricks large-scale wall specimens were constructed and tested to failure. The dimensions of the walls were 2.64 m · 2.64 m (8.67 ft · 8.67 ft) and one brick wide. Large wall dimensions were selected to avoid scale effects and to reflect the actual performance of a weak masonry wall under out-of-plane flexural loading conditions. Table 1 describes the wall specimens evaluated in this study. As mentioned earlier, the International Code Council-Evaluation Service (ICCES) Acceptance Criteria AC 125 [19] procedures were followed for all tests.

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4.2. Mortar

Standard Type-S mortar was used in constructing these walls. The mortar was mixed to the proportion specification of ASTM C-270 Standard. The average compressive strength of the mortar was obtained by testing six 50.8 mm (200 ) diameter · 101.6 mm (400 ) high cylinders taken from the same batch used in fabricating the wall specimens. The average strength ð f m0 Þ of the mortar on the day of the tests was 21.37 MPa (3100 psi). 4.3. Red bricks

Common red clay bricks (Castaic) readily available from building suppliers were used. The nominal dimensions were 20.32 cm (8 in.) · 10.16 cm (4 in.) · 5.72 cm (214 in.). Two types of tests were conducted on the red bricks: (i) a unit brick compression test to determine the ultimate compressive strength (refer to Fig. 2), and (ii) a prism test (refer to Fig. 3) to determine the combined compressive strength of

Table 1 Description of wall specimens Test ID

Specimen’s description

Control unreinforced

WCONT-U

As-built wall no composites

Retrofit: carbon/epoxy

WC-RET-02 WC-RET-090

2 Plies of unidirectional carbon/epoxy laminate parallel to edge supports [0]2 1 Ply of unidirectional carbon/epoxy in each orthogonal direction (perpendicular to edge supports direction) [0/90]1

Retrofit E-glass/epoxy

WE- RET-02

3 Plies of unidirectional E-glass/epoxy laminate parallel to edge supports [0]3

Fig. 2. Compression test for unit red clay bricks.

Fig. 3. Setup and results of the prism test.

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the unit bricks and mortar. The average compressive strength obtained from unit brick tests is 25.00 MPa [3.63 ksi]. A lower average strength value of 16.0 MPa [2.33 ksi] was obtained from the prism tests. 4.4. Composite materials

Both carbon/epoxy and E-glass/epoxy composite systems were evaluated. Specifications for each system component including fibers and epoxies, and certifications for random sampling in accordance with requirements of ICC-ES (AC85 [20]) were obtained. This procedure is critical to ensure that all the off-the-shelf composites materials used in laboratory testing are true representation of materials to be used in the field. 4.5. Test setup

All specimens were tested in a water-bag structural frame as shown in Fig. 4. The hydraulic pressure load

was applied uniformly to each specimen until ultimate failure occurred (refer to Fig. 5). The applied pressure was controlled via a data acquisition computer program connected directly to the loading frame. The load followed a cyclic loading/unloading regime, which was designed specifically for these tests. In all tests a loading rate of 3.45 kPa/min (72 psf/min) was used. The wall specimens were simply supported only on two parallel sides while the other two sides were unsupported (refer to Fig. 4). A calibrated pressure transducer was used to control and measure the applied water pressure for all tests. All data was monitored and recorded using a computerized data acquisition system, which also provides real-time monitoring of data during testing. Deflection and strain data were measured using linear variable differential transducers (LVDTs) and electronic strain gages, respectively. Deflection and strain data were collected using a computerized data acquisition system. Stress/strain (r/e) and load/deflection (P /d) curves were developed for each specimen and both localized and ultimate failure modes were recorded and then analyzed. Fig. 6 shows the boundary conditions and the locations of different deflection and strain gages. 4.6. Strengthening schemes

Three categories of walls were tested: (i) two (as-built) control walls, (ii) two carbon/epoxy retrofitted walls (WC-RET-02 and WC-RET-0/90) with different lamination schedules, and (iii) an E-glass/epoxy retrofitted wall (WE-RET-02). The following sections summarize the findings of each test group. 4.7. Control (as built) tests

Fig. 4. Out-of-plane water bag wall test rig.

At first, an as built, unstrengthened brick wall specimen was tested to provide a baseline for comparison with other

Fig. 5. Typical hydrostatic pressure loading of wall specimens.


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Fig. 6. Locations of deflection and strain gages for strengthened wall specimens.

FRP strengthened specimens. No external composite reinforcement is used (except of two 76.2 mm (3 00 ) single crosslaminates adhered to the compression surface to avoid failure while transporting the specimen to the test rig). The control wall was subjected to several cycles of uniform pressure. The self-weight of the wall specimen was subtracted from the values of the applied uniform pressures. As the out-of-plane pressure was applied, the ‘‘asbuilt’’ wall specimen exhibited a near-linear behavior up to a pressure intensity of 3.30 kPa (69.10 psf), after which behavior became non-linear until failure occurred. The ultimate load capacity of this specimen was 6.5 kPa (136 psf), with an associated mid-height deflection at the maximum load of 45.41 mm (1.7900 ). After reaching this ultimate load, rapid stiffness and strength degradations were observed, and the ultimate deflection at the total collapse was about 77.30 mm (3.0400 ). The failure initiated around mid-span and started with development of a mortar line crack that propagated across the width of the specimen. As the load increased, the size of the cracks increased and a total sudden collapse occurred. Figs. 7 and 8 show the ultimate failure of the as-built wall specimen and the pressure/deflection behavior measured at different locations, respectively.

Fig. 7. Ultimate failure mode of the as-built specimen.

4.8. Strengthened walls tests

In order to evaluate the FRP composite systems’ effectiveness in strengthening unreinforced brick walls, three fullscale retrofitted specimens with different composite systems and fiber architectures, described earlier, were evaluated. In

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designing the strengthened specimens, the limit-states were based on the composites systems mechanical properties, especially the rupture strain of both carbon/epoxy ðecu ¼ 1:25%Þ and the E-glass/epoxy ð eEu ¼ 2:20%Þ, as well as the extreme brick fiber compressive strain for strain compatibility requirements. 4.8.1. Unreinforced brick walls retrofitted with carbon/epoxy composite laminates

A total of two unreinforced/undamaged specimens fabricated at the same time, with same materials, and by the same contractor as for the as-built specimen, were instrumented

and tested to failure under the same out-of-plane uniformly distributed loading condition. Two fiber architectures were evaluated: (i) two unidirectional laminates covering the entire wall (0)2, and (ii) two cross-ply laminates (0/90)1. The reason of using the second fiber architecture is that in general retrofit cases, the wall will be exposed to both inplane as well as to out-of-plane seismic loads, and it is not obvious what would be the effect of this multidirectional fiber architecture on the overall performance of the retrofitted wall. A sufficient gap was provided between the wall edges and the supporting steel channels to prevent any possibility of developing ‘‘arching action’’ in the test specimens.

Fig. 8. Pressure–deflection curves for the as-built brick wall.

Fig. 9. Pressure/deflection curves for unidirectional [0]2 carbon/epoxy strengthened brick wall specimen (WC-RET-02).

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A.S. Mosallam / Composites: Part B 38 (2007) 559–574 2500

Carbon/Epoxy (0o)2

10000

2000

7500 ) f s 1500 p ( e r u s s e 1000 r P

SG-1

5000

SG-4 SG-5

) a P k ( e r u s s e r P

SG-6 SG-7

500

2500

SG-8

0 0.000

0.001

0.002

0.003

0.004

0.005

0.006

0.007

0 0.008

Strain

Fig. 10. Pressure/strain curves for the unidirectional [0]2 carbon/epoxy strengthened brick wall specimen (WC-RET-02).

4.9. Cyclic and ultimate behavior of [0 ] 2 carbon/epoxy strengthened unreinforced brick wall specimen (WC-RET02)

The wall specimen was subjected to several loading/ unloading cycles up to 39.76 kPa (830.45 psf), after which a ramp load (3.45 kPa/min or 72 psf/min) was applied up to the ultimate failure of the wall specimen. The first cracking sound was heard at a load level of about 27.17 kPa (567.47 psf). This can be attributed to stretching of the fibers due to the unevenness of the wall surface and the mortar lines as well the stiffness incompatibility of the two materials (composites/bricks). The behavior was linear from a load level of about 39.76 kPa (830.45 psf) and up to failure as shown in Fig. 9. From this figure, one can see that both LVDT 3 and LVDT 4 reading were identical (Refer to Fig. 6 for gages’ locations.) which confirms the accuracy of the deflection measurements and the symmetrically applied loading condition. The ultimate laminate strain at failure was 0.71% as shown in Fig. 10. This strain is 57% of the rupture strain of the carbon/epoxy composite system (eu = 1.25%). The ultimate load capacity of this specimen was 74.43 kPa (1554 psf) which is 12 times the capacity of the as-built specimen. The associated mid-height out-of-plane deflection was 87 mm (3.422 in.). The ultimate failure mode was a combination of a compressive failure of the bricks followed by a cohesive failure of the carbon epoxy laminates as shown in Figs. 11 and 12, respectively. 4.10. Cyclic and ultimate behavior of [0 /90]1 carbon/epoxy strengthened unreinforced brick wall specimen (WC-RET090)

There were two main reasons behind the selection of this fiber architecture. Although, the wall resists out-of-plane uniform pressure loading in one-way action (in this study,

Fig. 11. The combined failure mode of brick compression failure and laminate cohesive failure.

walls were supported at only two parallel sides as shown in Fig. 6), in the common field application both in-plane and out-of-plane reinforcements for an unreinforced wall are typically required. For this reason, it was decided to use this cross-ply [0/90] lay-up in order to: (i) investigate the one-way, out-of-plane flexural response of the brick wall specimen when strengthened with one single ‘‘effective’’ laminate of carbon/epoxy system, and (ii) to evaluate the effect of the presence of the 90  laminate to both the service performance and ultimate failure mode of brick walls. From first glance, one may expect that the 90 laminate may not contribute to both the structural performance of the one-way load-resisting path. However, test results obtained from this study indicated that this scheme may alter the ultimate mode of failure by suppressing the expected longitudinal separation of the ‘‘effective’’ laminates (in the 0-direction) and force these laminate to work

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Fig. 12. Ultimate failure mode of the unidirectional laminated (only on one side) red brick wall specimen.

together. The contribution of the 90-ply was shown to be effective and is considered to be a contributing factor in determining the ultimate failure mode of this specimen. The cross-ply actually acts as a cross-support which forces the 0-degree laminated strips to deform as a single wide laminate. This prevents the 0-degree separation between the unidirectional laminates that was observed in specimen strengthened with two plies of unidirectional carbon/epoxy composites described earlier (refer to Fig. 12). This specimen achieved the highest performance, with respect to the resulting ductile failure that was observed for this specimen. Test results indicated that, even with a

single ‘‘effective’’ laminate of carbon/epoxy system, an appreciable increase in the wall strength was achieved. For example, the ultimate capacity of this specimen was 60.58 kPa (1265 psf) compared to 6.52 kPa (136.2 psf), and 74.43 kPa (1554 psf) of as-built wall, and the wall specimen strengthened with two unidirectional layers carbon/ epoxy system, respectively. This ultimate capacity is about 81% of specimen WC-RET-02 and 9.22 times the strength of the as-built specimen. The maximum mid-height deflection at failure was 98 mm (3.859 in.). Fig. 13 shows the pressure/deflection behavior of this specimen. The strain at failure of this specimen was 1%,

Fig. 13. Cyclic performance of the cross-ply carbon/epoxy strengthened brick wall specimen (WC-RET-090).


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Fig. 14. Pressure/strain curves for the cross-ply carbon/epoxy strengthened brick wall specimen (WC-RET-090).

which is about 83% of the measured rupture strain of the carbon/epoxy system. This is another indication of the merit of using the cross-ply is that it succeeded in increasing the efficiency of the external FRP composite reinforcement system (the ultimate strain is 16.9% higher than specimen WC-RET-02). Fig. 14 shows that pressure/strain curves for the composite laminate of specimen WC-RET-090. The ultimate failure mode was similar to the two-ply unidirectional wall specimen, which is a combination of

compression failure of the bricks and a cohesive failure of the composite laminates. However, and due to the suppressing action of the cross-ply, the failure was more ductile and no longitudinal separation between the laminated strips (parallel to the unsupported free edges) was observed. Figs. 15 and 16 show the ultimate failure modes of this specimen.

Fig. 15. Ultimate failure mode of the cross-ply ‘‘effective’’ laminate on one side of brick wall specimen (WC-RET-090).

Fig. 16. Compression failure of the red bricks at ultimate load of cross-ply carbon/epoxy strengthened wall specimen WC-RET-090.

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4.11. Cyclic and ultimate behavior of [0]3 E-glass/epoxy strengthened brick wall specimen (WE-RET-02)

For this wall specimen, a total of three unidirectional plies of E-glass/epoxy were applied covering the entire tension surface of the brick wall specimen. The unidirectional fibers were aligned parallel to the free-edges of the unreinforced wall specimen (refer to Fig. 6). The behavior of this specimen was similar to the retrofitted specimen WC-RET-02. Figs. 17 and 18 show the loading/unloading pressure/deflection behavior of this specimen at service loading conditions (In this paper, service load is defined

as 40% of the ultimate capacity of the wall specimen which equals to 30.12 kPa (628 psf). At this pressure load level, fine hair cracks were observed at three locations.). For the ultimate load tests, specimens were subjected to a constant ramp load up to failure. The maximum central deflection at failure was 91.7 mm (3.61200 ). The ultimate capacity of this wall was 75.29 kPa (1572 psf) as shown in Fig. 19. This capacity is 11.54 times the out-of-plane ultimate capacity of the control, unstrengthened wall specimen. The strain at failure of the mid-height surface laminate was 1.07% (refer to Fig. 20), which translates to about 48% of the experimentally obtained rupture strain of the E-glass/epoxy FRP

Fig. 17. Pressure/deflection curves of the unidirectional [0]3 E-glass/epoxy strengthened unreinforced brick wall specimen at low cyclic load (WE-RET02).

800

3500

E-glass/Epoxy - Service Load 700 3000 600 2500 ) 500 f s p ( e r 400 u s s e r 300 P

2000

SG-1 SG-3 SG-4

1500


SG-5 SG-6

200

1000

SG-7 SG-8

100

0 0.0000

500

0 0.0005

0.0010

0.0015

0.0020

Strain

Fig. 18. Pressure/strain curves unidirectional [0]3 E-glass/epoxy strengthened unreinforced brick wall specimen at low cyclic load level (WE-RET-02).

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A.S. Mosallam / Composites: Part B 38 (2007) 559–574 Deflection (mm) 0 2000

10

20

30

40

50

60

70

80

90

E-glass/Epoxy - Ultimate Load

100

8000

1500 6000 ) a

) f s p ( e r 1000 u s s e r P

4000 Deflection-1

P k ( e r u s s e r P

Deflection-2

500

Deflection-3 Deflection-4

2000

Deflection-5

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0 4.0

Deflection (inch)

Fig. 19. Pressure/deflection curves unidirectional [0]3 E-glass/epoxy strengthened brick wall test specimen at high load levels (WE-RET-02).

2000

E-glass/Epoxy - Ultimate Load 8000

1500 ) f s p ( e r u 1000 s s e r P

6000

SG-1 SG-3 SG-4 SG-6 SG-7

500

0 0.000

4000

SG-5


SG-8

0.002

0.004

0.006

0.008

0.010

2000

0 0.012

Strain Fig. 20. Pressure/strain curves unidirectional [0]3 E-glass/epoxy strengthened unreinforced wall specimen at high load levels (WE-RET-02).

composite system. Although the E-glass/epoxy inherently exhibits lower stiffness properties compared to carbon/ epoxy-type laminates, the average stiffness increase in the linear range of this wall, as compared to the as-built specimen, was about 60% higher. The failure mode was different from the carbon/epoxy specimens. For this specimen (in addition to the combined mode of failure of compressive failure of the bricks followed by cohesive failure of the E-glass/epoxy laminate that was observed in all preceding tests), a tensile fracture of the laminate did occur as shown in Figs. 21 and 22. This can be attributed to the relatively lower tensile strength of E-glass/epoxy laminates as compared to carbon/epoxy laminates.

5. Theoretical modeling

An analytical model was developed to predict the ultimate load of the retrofitted specimens. The analytical model used in this study is based on simple section analysis procedures similar to that used for analyzing reinforced concrete beams. However, new parameters have been used for masonry wall based on available experimental data (e.g. [21,22]). The first part of the analysis is to define the material properties. The stress–strain curve for brick-mortar blocks under compression is as shown in Fig. 23. The curve consists of two distinct regions: a parabolic relationship up to the maximum compressive strength, f m0 , and a linear

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 For emo < em < emu: f m ¼ f m0  E d ðem  emo Þ

ð5Þ

0:5 f m0 E d ¼ ðemu  emo Þ

ð6Þ

The parameters of the previous stress–strain curve are given as [21,22]: f m0 ¼ 31 MPa (4500 psi); emo = 0.002; emu = 0.0035; E m = 19.28 GPa (2.8 · 106 psi); and f mf ¼ 0:5 f m0 . Based on experimental evidences, it is appropriate to assume the FRP composites to be linear elastic up to failure as shown in Fig. 24. The properties for both carbon/ epoxy and E-glass/epoxy systems used in this study are presented in Table 2. The section analysis procedures adopted in this study are based on the following assumptions:  Tensile strength of the brick-mortar blocks is ignored.  Tensile resistance of the FRP laminates can be neglected in the transverse direction.  The area of the FRP laminates is enough for the failure of the specimen to be due to masonry crushing rather than fiber fracture.  Plane section before bending remains plane after bending, and hence a linear strain distribution can be assumed along the section.

Fig. 21. Large mid-height deflection of unidirectional [0]3 E-glass/epoxy strengthened brick wall specimen (WE-RET-02).

In order to perform the section analysis, it is necessary to develop parameters describing the equivalent rectangular stress block shown in Fig. 25. These parameters can be determined by integrating the stress–strain curve for brick-mortar blocks in compression, as follows: emu

descending branch up to the ultimate compressive strain, emu. The first region of the stress–strain curve is assumed polynomial in the form: f m ¼

A enm

þ Bem þ C

ð1Þ

The four unknowns in Eq. (1) are determined from the following boundary conditions: ðiÞ ðiiÞ ðiiiÞ ðivÞ

at em ¼ 0:0 f m ¼ f m0 at em ¼ e mo d f m =dem ¼ E m at em ¼ 0:0 d f m =dem ¼ 0:0 at em ¼ e mo

f m ¼ 0:0

ð2Þ

The equations of the stress–strain curve have been determined to be:  For 0 < e m < emo: f c ¼ E m em n ¼

"   # 1

em 1  n emo

E m emo E m emo  f m0

" R R R

b ¼ 2 1 

0

emu

f m em dem emu

0

f m dem

#

¼ 0:88

emu

c ¼

f m dem

b f m0 emu

¼ 0:8

ð4Þ

ð8Þ

6. Numerical example

In the following example, the proposed analytical approach is used to predict the out-of-plane capacity of a red brick wall strengthened with two unidirectional plies of carbon/epoxy composite system. Dimensions, boundary conditions, loading pattern, composite lay-up and properties are identical to those used for wall specimen WCRET-02 evaluated in this study (refer to Fig. 6). The following are the step-by-step analytical procedures for predicting the flexural capacity of this wall.

n1

ð3Þ

ð7Þ

6.1. Strengthened wall information

 Wall dimensions: 2.64 m · 2.64 m (104 00 · 104 00 ).  Brick wall thickness: 101.6 mm (4 00 ).

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Fig. 22. Ultimate combined failure mode of unidirectional [0]3 E-glass/epoxy strengthened unreinforced brick wall specimen (WE-RET-02).

f m s s e r / t S f m e v i s s e r f mf p m o C

Ed

m mo

mu

m

Compressive Strain

Fig. 23. Stress–strain model for brick-mortar blocks in compression.

Fig. 24. Stress–strain model for typical FRP laminates.

Table 2 Properties of composite materials Composite system Unidirectional carbon/epoxy Unidirectional E-glass/epoxy

t p, mm [in.]

Ply thickness,

On-axis tensile modulus, E j , GPa [·106 psi]

0.584 [0.023]

103.4 [15.06]

1.143 [0.045]

18.47 [2.679]

 Composite system: Carbon/epoxy (CFRP) wet lay-up system (refer to Table 2).

On-axis tensile strength, f ju, MPa [·103 psi]

On-axis tensile ultimate strain, e u (%)

1245.83 [180.7]

1.25

424.70 [61.6]

2.20

 CFRP ply unit thickness = t p = 0.584 mm (0.0200 ).  Number of unidirectional plies = n = 2.

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Fig. 25. Stress and strain distribution for section analysis.

 Total thickness of CFRP laminate = t j = t p · n.  Ultimate CFRP on-axis tensile strain = e j = 1.25% (refer to Table 2).  CFRP on-axis tensile modulus = E j = 103.4 GPa (15.06 Msi).  CFRP on-axis tensile strength = f ju = 1245.83 MPa (180.7 ksi).  Boundary conditions: Simply supported on the two opposing sides (sides perpendicular to fiber directions as shown in Fig. 6). 6.2. Analytical procedures

1. Calculation of neutral axis depth (refer to Fig. 25): h = 400 + 0.04600 /2 = 4.023 in. (102.2 mm), a = bc = 0.88c, C ¼ c f m0 ab = 0.8 · 3.629 ksi · a · 10400 = 301.93a, T = A j f j = A j E j e j = 10400 · (2 · 0.02300 ) · 15,060 · e j = 72,047e j . From strain compatibility: e j = 0.0035(h/c  1) = 0.01239/a  0.0035, T = 892,663a  252.17. From equilibrium: C = T or 301.93a = T = 892,663a  252.17 from which: a = 1.3500 (34.34 mm). 2. Check of CFRP allowable strain: e j = 0.00567, e ju = f ju/E j = 0.0125 > e j ok. Thus, failure is due to masonry crushing rather than fiber fracture. 3. Calculation of ultimate moment and maximum load: M u = ultimate flexural capacity = c f m0 abðh  a=2Þ ¼ A j f ju ðh  a=2Þ, or M u = 1366.22 kip-in. (154.36 N m), wu = ultimate unit load = 8M u/L2 = 1.01 kip/in. (177.055 kN/m), P u = ultimate load capacity = 1.011 kip/in. · 104 00 = 105.14 kip (467.44 kN), uniform pressure = 105.14 kip s/ pu = ultimate 00 2 (104 ) = 1399.2 psf (66.94 kPa). From this simple analysis, the predicted ultimate pressure was slightly less than the ultimate pressure obtained

Table 3 Summary of theoretical analysis for retrofitted specimens Specimen configuration

Experimental maximum load, Experimental , p ultimate kPa (psf)

Theoretical p Theortical ultimate maximum Experimental p ultimate load, p Theoretical ultimate , kPa (psf)

(0)2 Carbon/ epoxy (0/90)1 Carbon/epoxy (0)3 E-glass/ epoxy

74.4 (1554)

67.0 (1397)

0.90

60.6 (1265)

53.0 (1107)

0.87

75.3 (1572)

55.0 (1148)

0.73

¼ 1554 psf =74:43 kPaÞ. However, from the actual test ð p Exp u the 10% deviation from the experimental ultimate pressure is in the conservative side which is desirable. Similar procedures were used to predict the other two specimens evaluated in this study. Table 3 presents a comparison between the predicted and experimentally obtained ultimate pressure values for the three strengthened walls evaluated in this study. 7. Conclusions and summary of results

The results of this study confirmed the effectiveness of both the E-glass/epoxy and carbon/epoxy FRP composite strengthening systems in upgrading the out-of-plane flexural structural performance of unreinforced brick walls. The strength gains resulted from adding the composite systems was appreciable as shown in Figs. 26–28. The failure modes of specimens were due to a combination of compression failure of red bricks followed by a cohesive failure as described earlier. The coupling effect of in-plane and outof-plane reinforcements is shown to have positive effects on both the out-of-plane capacity and the ductility of the retrofitted wall specimen. Furthermore, due to the suppressing action provided by the orthogonal ply (applied in the direction parallel to the support), end-of-strip longitudinal separation (parallel to unsupported free edges as shown in Fig. 12) observed in unidirectional reinforced wall (specimen WC-RET-02), was eliminated. Based on these observations, it is recommended that in order to achieve optimal out-of-plane performance of strengthened brick walls, cross-ply lamination schedule should be used. This will be satisfied in the case where both out-of-plane


573

Fig. 26. Ultimate capacity comparison for all wall specimens.

4.0

) h c 3.5 n i ( n o 3.0 i t c l e f e 2.5 D t h g 2.0 i e H d i 1.5 M e 1.0 t a m i t l 0.5 U 0.0

As-Built

E-glass (0) 3

Carbon (0) 2

Carbon (0/90)1

Fig. 27. Comparison of mid-height deflection at ultimate for all wall specimens.

and in-plane composite reinforcements are provided. However, if only out-of-plane reinforcement is required, it is recommended to add a lighter orthogonal ply (about 10– 15%) of the major flexural composite reinforcement demand. Additional research is needed in order to accurately determine the optimum percentage of orthogonal polymer composites reinforcements. The simple analytical approach developed in this study was successful in predicting the experimental ultimate out-of-plane flexural behavior of the walls. However, the accuracy of predicting the experimental results varied (refer to Table 3). For example, the highest correlation between analytical and experimental results was achieved for the wall specimen strengthened with unidirectional carbon/ epoxy composites. This can be attributed to the straightness of the carbon fabrics, used in this study, resulting in

a better representation of the composite mechanical properties that were used in the analytical modeling. On the other hand, the analytical results obtained for walls strengthened with cross-ply carbon/epoxy and E-glass/ epoxy composite laminates were relatively less accurate as compared to the experimental results. The possible reason behind this deviation for the cross-ply carbon/epoxy strengthened wall is the fact that the effect of the orthogonal ply was ignored in the analysis. As shown in Table 3, the analytical results for the E-glass/epoxy strengthened wall was about 30% less than the capacity observed in the test. This can be attributed to ignoring the kinking effect and fiber misalignment of the E-glass fabrics used in this study. More research is needed to develop analytical models capable of including both the effect of the orthogonal lamination as well as kinking effects.

574


1.0

) ( 0.9 n i a r 0.8 t s e 0.7 l i s n e T 0.6 e t a 0.5 n i m0.4 a L m0.3 u m i x 0.2 a M0.1 0.0

As-Built

E-glass (0) 3

Carbon (0) 2

Carbon (0/90) 1

Fig. 28. Comparison between mid-span tensile strains at ultimate load for all wall specimens.

Acknowledgement

The FRP materials used in the study was provided by Edge Structural Composites Inc. References [1] Mosallam AS. Composites in construction. Handbook of materials selection. NY, USA: John Wiley Publishing Co.; 2002, 53 p [Chapter 45]. [2] Schwegler G. Versta¨rken von Mauerwerk mit Faserverbundwerkstoffen in seismisch gefa¨hrdeten Zonen, Dissertation ETH Zu ¨ rich No. 10672, Published by Eidgeno ¨ ssische Materialpru ¨ fungs- und Forschungsanstalt, CH-8600 Du¨bendorf, as EMPA-Report Nr. 229, 1994. [3] Schwegler G. Versta ¨ rkung von Mauerwerkbauten mit CFK. Lamellen, Sonderdruck aus, Schweizer Ingenieur und Architekten, Nr. 44, 1996. [4] Gilstrap JM, Dolan CW. Out-of-plane bending of FRP-reinforced masonry walls. Compos Sci Technol J 1998;58:1277–84. [5] Albert LM, Elwi AE, Cheng JJ. Strengthening of unreinforced masonry walls using FRPs. ASCE J Compos Constr 2001;5(2):76–84. ´ r LP, Krawinkler H. Masonry [6] Ganz HR, Kiss RM, Jai J, Kolla strengthened with fiber reinforced plastics subjected to combined bending and compression. Part I – Model. J Compos Mater 2001. [7] Kiss RM, Kolla´r LP, Jai J, Krawinkler H. Masonry strengthened with FRP subjected to combined bending and compression. Part II: Test results and model predictions. J Compos Mater 2002;36(9): 1049–62. [8] Velazquez-Dimas J, Ehsani MR, Saadatmanesh H. Cyclic behavior of retrofitted URM wall. In: Proceedings of the 2nd int conf on composite in infrastructure (ICCI), Tucson, Arizona, 1998. [9] Kuzik MD, Elwi AE, Cheng JJ. Cyclic flexure tests of masonry walls reinforced with glass fiber reinforced polymer sheets. ASCE J Compos Constr 2003;7(1):20–30. [10] Tan KH, Patoary MKH. Strengthening of masonry walls against outof-plane loads using fiber-reinforced polymer reinforcement. ASCE J Compos Constr 2004;8(1):79–87.

[11] Al-Salloum YA, Almusallam TH. Load capacity of concrete masonry block walls strengthened with epoxy-bonded GFRP sheets. J Compos Mater 2005;39(19):1719–44. [12] Turco V, Secondin S, Morbin A, Valluzzi MR, Modena C. Flexural and shear strengthening of un-reinforced masonry with FRP bars. Compos Sci Technol 2006;66:289–96. [13] El-Dakhakhni WW, Hamid AA, Hakam ZHR, Elgaaly M. Hazard mitigation and strengthening of unreinforced masonry walls using composites. Compos Struct 2006;73:458–77. [14] Hamoush SA, McGinley MW, Mlakar P, Scott D, Murray K. Outof-plane strengthening of masonry walls with reinforced composites. ASCE J Compos Constr 2001;5(3):139–45. [15] Hamilton III HR, Dolan CW. Flexural capacity of glass FRP strengthened concrete masonry walls. ASCE J Compos Constr 2001;5(3):170–8. [16] Laursen PT, Seible F, Hegemier GA. Seismic retrofit and repair of reinforced concrete with carbon overlays. Report no. SSRP-95101, University of California, San Diego, 1995. [17] Ghobarah A, Galal K. Out-of-plane strengthening of unreinforced masonry walls with openings. ASCE J Compos Constr 2004;8(4):298–305. [18] Korany Y, Drysdale R. Rehabilitation of masonry walls using unobtrusive FRP techniques for enhanced out-of-plane seismic resistance. J Compos Constr 2006;10(3):213–22. [19] International Code Council-Evaluation Service (ICC-ES). Acceptance criteria for concrete and reinforced and unreinforced masonry strengtheningusing fiber-reinforced polymer (FRP)compositesystems (AC125), July 2003 [http://icc-es.org/criteria/pdf_files/ac125.pdf ]. [20] International Code Council-Evaluation Service (ICC-ES). Acceptance criteria for test reports (AC85), July 2003 [http://icc-es.org/ criteria/pdf_files/ac85.pdf ]. [21] Fattal SG, Gattaneo LE. Structure performance of masonry walls under compression and flexure, National Bureau of Standards (NIST), Washington, DC, 1976. [22] Triantafillou TC. Strengthening of masonry structures using epoxybonded FRP laminates. ASCE J Compos Constr 1998;2(2).

Out of Plane Flexural Behavior of Brick Wall_2006

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